The invention relates to a sensor arrangement comprising a carrier substrate and a ferroelectric layer, to a method for producing the sensor arrangement, and to the use of the sensor arrangement.
It is known that polarization Pi (Asm−2) can be induced in an insulating, polarizable material, referred to as a dielectric, by way of an external electric field E (Vm−1). Among dielectrics, a displacement of the charge, and thus electrical polarization of the material, is brought about in piezoelectric materials not only by external electric fields, but also by an external mechanical deformation caused by pressure, tension or torsion. The positive and negative lattice components are displaced as a result of the deformation so that an electric dipole moment is created, whereby apparent charges are induced on the surface of the outwardly neutral crystal. The term pyroelectricity collectively refers to those materials among piezoelectrics that have electric dipole moments even in the absence of an external electric field, which are caused based on distortions in the crystal lattice and the attendant displacement of charge centroids, thus bringing about electric polarization of the crystal. These substances are thus spontaneously polarized even without an electric field. Finally, the term ferroelectricity collectively refers to those substances having an electric dipole moment which change the direction of spontaneous polarization when an external electric field is applied. This phenomenon disappears above the material-dependent Curie temperature, and the material transitions into the paraelectric state. This transition is reversible, which is to say a phase transition with a structural change takes place when a drop below the Curie temperature occurs, and the material again transitions into the ferroelectric state. Permittivity, which is to say also the change in permittivity with the temperature, is typically the greatest in the range of the transition. A reversible change in permittivity which is as great as possible is thus achieved in the temperature range directly above the phase transition.
The majority of ferroelectrics are oxides. The best-known ferroelectrics are ion crystals having a perovskite structure, such as BaTiO3. Some materials exhibit ferroelectric properties only in thin layers, for example SrTiO3. Ferroelectric layers are essentially used in integrated circuits and in mobile radio communication technology.
It is known from more recent developments on ferroelectrics (R. Wördenweber, E. Hollmann, R. Ott, T. Hürtgen, Kai Keong Lee (2009). Improved ferroelectricity of strained SrTiO3 thin films on sapphire. J Electroceram 22:363-368) to strain thin films made of ferroelectric SrTiO3 (STO) by way of epitaxial growth and application of the lattice parameters on CeO2 buffered sapphire, for example. The dielectric properties of the strained STO were ascertained by a capacitor on the layer.
The operating principle of pressure and bending sensors is typically based on the conversion of the parameter to be measured into an electrical signal. This can take place directly or indirectly. Pressure and bending sensors can be used to directly measure pressure or deflection and to indirectly determine other parameters, such as the temperature, the flow or the position.
Depending on the use, the desired measuring accuracy and costs, (piezo)resistive sensors, piezoelectric sensors, inductive sensors, capacitive sensors and optical sensors are employed, for example. Often, combinations of these sensors are employed, such as in a Golay cell.
A piezoelectric pressure sensor is characterized in that a charge separation is induced in the crystal having polar axes by way of the pressure to be measured, and the charge separation induces an electric voltage. This state, also known as piezoelectric effect, thus causes ions to be displaced due to pressure in the interior of the crystal, forming an electric charge on the surface that is proportional to the mechanically exerted force. The charge is converted into electric voltage proportional thereto, such as by way of an amplifier. With piezoresistive sensors, in contrast, the resistivity of the materials changes as long as they are subjected to a tensile load or pressure load. This effect also occurs in crystals without a polar axis, for example in semiconductors, such as silicon.
A general problem encountered with piezoelectric pressure sensors and the piezoresistive sensor arrangements used in strain gauges is that these have a comparatively complex design and are therefore costly. The sensitivity of these sensors likewise has some room for improvement.
Another drawback is that pyroelectricity often causes interfering artifacts in the technologically relevant field of application, which are superimposed on the piezoelectric effects that are actually of interest. Pressure sensors made of pyroelectric materials can indicate false positive signals since signals occur under heating, while no change in pressure is present at all.